1 Field of the Invention
The present invention relates to display control technology, and more particularly to effective techniques for improving a color graphic display system. It is effectively utilized for, e.g., a drawing processor or graphic controller which has a pattern data drawing function for a video memory that holds video data in conformity with a bit map system.
2 Description of the Prior Art
In recent years, in a graphic display system having a raster scanning type CRT display device by which a complicated pattern is displayed in colors, there has come into use a drawing processor or a graphic controller of multiprocessor architecture which has the function of drawing video data for a video memory (frame buffer) of the bit map type wherein the video data items are formed on the basis of pattern information stored in a main memory on a host side and are held in one-to-one correspondence with pixels on the display screen of the CRT. Some of such graphic controllers have the function of superposing two pictures by the use of the functions of a logical operation unit ("NIKKEI ELECTRONICS" published by Nikkei McGraw-Hill Kabushiki-Kaisha, dated May 21, 1984, pp. 221-254).